Screengrab taken from a militant video posted on YouTube of Reyaad Khan, killed in an RAF drone attack in Syria.
Photograph: YouTube/PA

It is difficult to exaggerate the significance of the targeted killing by RAF drones of two Britons in Syria. It has serious implications , political, constitutional, legal - and for the future conduct of military operations.

One key unanswered question is why David Cameron and Michael Fallon, on the advice of the security and intelligence agencies, considered Isis fighter Reyaad Khan posed such a threat to the British public that there was no alternative but to kill him.

Ruhul Amin was killed in the same strike, on 21 August. A third Briton, Junaid Hussain, was killed by a US drone in a joint US-UK operation three days later.

We need to be told why they were killed in August when the targets Khan and Hussain were said to be plotting to attack were VE Day commemorations in May and Armed Forces Day in June.

“There are other terrorists involved in other plots that may come to fruition over the next few weeks and months and we wouldn’t hesitate to take similar action again”, Michael Fallon, the defence secretary, told BBC Radio 4’s Today programme on Tuesday morning.

He added: “ IF THERE WAS NO OTHER WAY OF PREVENTING THESE ATTACKS (my emphasis) we wouldn’t hesitate to do it again”, he added.

Yet if the intelligence agencies knew about the plots, why did not MI5 or the police take action here?

Under Article 51 of the UN Charter, every country has the right of self-defence. “Under the right of self-defence, any armed attack [against the UK] would have to be imminent or actual;”, Phillipe Sands QC, professor of law at University College London, told the Guardian. Government officials say they were “planning” an attack.

Under the so-called Caroline Test, relating to a 19th century case and referred to by legal commentators on Tuesday, the need for pre-emptive self-defence must be “instant, overwhelming, and leaving no choice of means, and no moment for deliberation”.

Michael Clarke, director general of the London-based thinktank, RUSI, says: “The point is not so much that this man (Khan) was British but that he was targeted in an area that the UK does not currently regard, legally, as an operational theatre of war for UK forces.”.

He adds: “The government insisted that, unlike CIA drones, they were never used for targeted assassinations in territories where we were not militarily engaged. This would have been in contradiction to the UK’s criminal justice approach to counter terrorism. The prime minister made it clear that the legal justification in this case was explicitly one of ‘self-defence’. He may have to say more about that to justify the sceptics.”

Echoing the thoughts of many, Clarke continued: “It may turn out to be a Downing Street high-risk strategy to log-roll a controversial issue through parliament, throw a spanner into the works of the new Labour leadership when the winner is announced at the end of this week, and incidentally bury a switch in legal policy inside the complexities of the current refugee crisis.”

The UK government has been at pains in the past to distant itself from the US use of drones, including a “kill list” of selected targets. Britain now has its own “kill list”.

A University of Birmingham review headed by Sir David Omand, former GCHQ director and government top security and intelligence adviser, said last year Britain should lead a campaign for an international ban on development of autonomous “killer robots” but existing armed drone technology posed no “convincing ethical” problems, according to a policy commission headed by a former director of GCHQ.

The report, The Security Impact of Drones: Challenges and Opportunities for the UK, concluded that the threshold for the use of force would not be lowered by the use of drones but warned that it depended on “parliament playing its proper oversight function”.

British officials needed to be careful, the commission warned, that intelligence cooperation with the US did not involve British troops or officials in illegal activity.

Over 1,000 high-profile artificial intelligence experts and leading researchers in July signed an open letter warning of a “military artificial intelligence arms race” and calling for a ban on “offensive autonomous weapons”.

At a UN conference in Geneva in April discussing the future of weaponry, including so-called “killer robots”, the UK opposed a ban on the development of autonomous weapons, despite calls from various pressure groups, including the Campaign to Stop Killer Robots.

There is a danger that drones will lower the threshold of violence - and extend the definition of “self defence” and “imminent” - at a time when the distinction between war and peace is getting blurred.

Military action is increasing intertwined with intelligence-gathering and therefore shrouded in ever more secrecy, while technology makes the use of lethal force much easier. And are we safer as a result?